This invention relates to the treatment of a solid carbonizable material, such as coal, shredded waste tires, comminuted municipal waste, sawdust and wood shavings and the like to convert such material into valuable products including combustible gases, liquid hydrocarbons and solid carbonaceous residues, and more particularly to the treatment of such a carbonizable material in a moving bed pyrolysis process and apparatus within a substantially horizontal high temperature zone.
The concept of recovering combustible gaseous, liquid hydrocarbons and usable coarbonaceous solid residues from coal and from municipal and industrial wastes has received increased attention with the rising costs of solid, liquid and gaseous fossil fuels and concomitant problems with conventional means of disposal or utilization of solid wastes. With respect to solid wastes, the intrinsic energy therein is a valuable resource which need not be wasted.
Solid waste disposal in the U.S. has become a major problem. At this time, approximately 8 pounds of solid waste is generated per capita per day. The present methods for disposing of municipal wastes are wholly inadequate with about 90 percent of such collected wastes being disposed of in landfill areas. Municipal incineration of wastes has been highly curtailed because of high costs of providing and operating incineration facilities which meet air pollution standards. Of particular concern in solid waste management is the handling of nearly 200 million waste rubber tires which are discarded each year in the U.S. If burned in municipal incinerators, rubber tires give off large quantities of unburned hydrocarbons with the smoke generated by tire combustion being highly visible and noxious requiring expensive control equipment to meet air pollution regulations. Tire disposal is also troublesome in sanitary landfills since whole tires, compacted in bulk into landfills, spring back to their former shape and tend to work-up during settling of the fill. Ultimately, whole tires emerge at the surface where their appearance is objectionable and offer refuge for rats and other disease carriers. Furthermore, tires are highly resistant to natural decomposition making them a permanent and ever increasing solid waste problem. The heat value per pound of tires, which is equal to or greater than that of coal, has suggested the feasbility of waste tires as a fuel, either in a supplementary role or as a sole source of energy, and several pilot studies of such use are underway at the present time.
Advocates of the development of energy sources other than gaseous and liquid fossil fuels have begun to re-examine coal gasification processes in view of the substantial reserves of such form of fossil fuels and the increasing costs of transporting coal to ultimate sites of utilization. Also, the wood processing industry is receiving increased attention as an untapped energy reservoir. Wood processing operations generate large volumes of wood and bark residues. Bark, chips, sawdust, sanding dust, shavings and trimmings comprise more than half of the volume of the original log.
There are three basic thermal techniques currently seen as feasible alternatives for producing energy from waste tires, municipal solid wastes and the like: (1) direct combustion of the raw or semi-processed solid waste in a grate-fired steam generator; (2) mechanically processing the organic fraction of the solid waste into a more readily usable form so that it can be burned by itself or as a supplement to other solid or liquid fossil fuels in a suspension or semi-suspension-fired steam generator; and (3) the pyrolysis or destructive distillation of the organic fraction of the solid waste into gaseous and/or liquid products which can be substituted for or used in conjunction with solid, liquid, or gaseous fossil fuels in steam generators or gas turbines. The thermal method of handling solid waste, carbonizing coal and wood residue which is gaining increased attention is that of deomposition of the solid material by pyrolysis. The term "pyrolysis" as used in the art means the chemical decomposition of a material by the action of heat in the absence of oxygen. Since the pyrolysis is performed in an essentially oxygen-free atmosphere, the solid waste material, coal or wood residue does not burn. Hence, the solid feed material decomposes into products which include steam, gases, liquids, tars and solid residues including coke, char and non-carbonizable materials as may have been contained in the feed material.
There appears to be two major reasons why solid waste, coal and wood conversion by pyrolysis is attracting attention. First, pyrolysis potentially makes refuse disposal a profitable operation and provides an attractive means of converting wood and solid fuels to liquid and gaseous energy sources. Substantially all of the end products (gases, liquids and solid residues) of pyrolytic conversion have economic value. Second, air-borne pollutants can be kept at very low levels.
Existing pyrolysis reactors tend to be bulky in size to accomodate large volumes of waste material to be processed and operate on a batch system of material treatment or have elaborate mechanisms to seal the reactor during its operation to preclude the escape of the reactor gases and/or the entry of oxygen or oxygen containing gases. The size of such reactors, their material feed devices and need for elaborate sealing systems obviously contribute to their high initial capital equipment cost and high cost of operation.